1. Field of the Invention
The invention relates to a device for crystallizing plastic material as well as to a method for treating plastic material.
2. Description of Related Art and Summary of the Invention
Such a method has for example been disclosed in EP-A-0 712 703. When such a method is implemented in practical applications, the finished product has some significant deficiencies:
the product is hydrolysed, i.e. partly decomposed and thus its quality is insufficient;
the finished product contains a relatively high dust fraction which not only means loss of material but also increased cost per unit of weight of the end product; and
the degree of crystallisation of the finished product is quite variable and the amorphous fraction is relatively high.
It is thus the object of the invention to create a product, e.g. bottle granulate made of PET or polyester material for tire cord etc., of improved quality with a low dust fraction and improved crystallinity. According to the invention this is achieved by the characterising portions of claim 1 or of claim 8.
The invention is based on the recognition that up to now, the production parameters have been selected in a somewhat carefree manner, with a bias to quick production. It has been found for example that slower heating during crystallisation with lower temperatures allows more gentle and economical production where in some cases even air can be used as a treatment medium instead of the normally used nitrogen.
It has also been found that the relatively high gas temperatures exceeding 195xc2x0 C. hitherto applied not only lead to product decomposition and thus to a drop in quality, but also cause excessive heat loss in the apparatus hitherto used. It is for example known from U.S. Pat. No. 5,119,570 to carry out precrystallisation and crystallisation in separate apparatus. However, separate apparatus also means a relatively high surface-to-volume ratio which promotes heat loss. While in the above-mentioned EP-A, precrystallisation and crystallisation are carried out in a mutual device, this does not result in optimisation either.
From these findings, the device of the invention was developed which results in a compact and economical design which at the same time results in minimal heat loss. The small heat losses also result in the conditions of crystallisation being better able to be kept under control, so that from this point of view too, there is no compulsion to apply high treatment temperatures. There are above all advantages in that the gas throughput for the compartments from a single gas source can be smaller than was the case so far and in that the overall height of the device itself can be kept lower, thus resulting in savings of space and cost. While a crystalliser with a rotation-symmetrical housing is known from CH-A-665 473, it is not designed or suitable for carrying out precrystallisation and crystallisation, i.e. it requires an additional device for a separate crystallisation step.
In the above mentioned EP-A the path of the plastic material flowing through the device is such that a high throughput of air is required so as to generate an aggregate fluidisation, and that the material is thrown above a free space situated above a partition wall, into the next compartment. By contrast, the invention preferably provides for the path to meander by arranging free space or free spaces and discharge aperture(s) at various levels in longitudinal section through the treatment space. While the known throwing-over results in significant differences in dwell times of the individual material components, with the measure according to the invention there is better control over the dwell time. It has been shown that in this way an excellent crystallisation degree with negligible amorphous fraction can be achieved. By contrast, in the case of a meandering path, the quantity of air flowing through can be reduced. In addition, in the case of free spaces or apertures located at the bottom, there is a certain division of the treated material according to its density even in a fluidised bed, with material of increased relative density being more likely to be in the bottom region. While the differences in density between amorphous and crystallised material in the fluidised bed are not very large, the fact that the denser material near the bottom is already crystallised out to a larger degree than the material fluidising further up, does have some significance. Consequently, from the lower free spaces, predominantly the material which has undergone certain crystallisation progress, is conveyed to the next compartment (or to the discharge aperture).
According to the above-mentioned findings, the method according to the invention starts off at relatively low temperatures which treat the material gently. While at first this requires somewhat more time, it does however bring about the preconditions for material of improved quality. The time lost in comparison to that of the state of the art can be saved by the subsequent shorter treatment time during preheating or precondensation. Preferably a fluidised bed design is used, so that there is no need for any agitator devices within the treatment space (compare U.S. Pat. Nos. 4,064,112 and 4,161,578), for it has been shown that said agitator devices cause significant losses due to strong dust formation. It must be pointed out that separation of condensation into precondensation and postcondensation with different treatment conditions has already been disclosed in U.S. Pat. No. 3,756,990 and that it is also the preferred way of implementing the method according to the present invention. This is also reflected in the final temperature of the material of at least 185xc2x0 C., preferably at least 200xc2x0 C., in particular approx. 220xc2x0 C.
In the aforesaid, among other things, the unevenness of the quality of the finished product has been criticised in the state of the art according to EP-A-0 712 703. Obviously the shortcomings of the crystallisation device that has been used, with random distribution of the dwell time, have been recognised in the state of the art which provides for subsequent heating in a container comprising rotating circulation devices. As has been found, it is precisely these circulation devices which are responsible for generating an excessive dust fraction. For this reason, with a view to a more gentle and controlled treatment of the material, the invention uses a different approach in that the crystallised material is brought into the shape of a rectangular bulk material stream of four-sided, in particular rectangular, cross-section of essentially even bulking across the cross-section; with treatment gas flowing from one side of the four-sided shape through said crystallised material. This means that as a result of this cross-sectional shape, the same conditions prevail for the gas along the entire inflow side, with the essentially even bulk density contributing its part. This can be improved still further in that the ratio of the rectangular sides of the cross-section of the bulk material stream is approximately 1:2 to 1:15, preferably ranging from 1:3 to 1:10, with the treatment gas being conducted through the bulk material stream from the larger side of the rectangle. In this way the method according to the invention differs from all those methods where the thicknesses of the bulk material stream and/or the gas flow conditions along the cross-section are different.
For gentle treatment, precrystallisation and crystallisation advantageously require a duration of between 10 and 80 minutes, preferably between 15 and 40 minutes, in particular between approx. 20 and 30 minutes. As mentioned above, according to the invention, subsequent heating up can be made more efficient by shortening this treatment step. According to the present invention this preferably takes place in that the heating following crystallisation, including precondensation, is carried out within a duration of 60 to 120 minutes, in particular approx. 90 minutes.